Field of the Invention
The present invention relates to methods for manufacturing a plugged honeycomb structure and plugged honeycomb structures. More particularly the present invention relates to a method for manufacturing a plugged honeycomb structure capable of manufacturing a plugged honeycomb structure with suppressed densification of a trapping layer, high heat capacity, low initial pressure loss, small increase in pressure loss during the deposition of particulate matter, and high trapping efficiency of particulate matter, and relates to a plugged honeycomb structure manufactured by such a manufacturing method.
Description of the Related Art
Gas emitted from an internal combustion engine, such as a diesel engine, or various types of combustion devices contain a lot of particulate substances (particulate matter (PM)) mainly containing soot. If this PM is discharged directly to the air, environment pollution will be caused, and so an exhaust system for exhaust gas includes a diesel particulate filter (DPF) to trap PM.
As such a DPF, a honeycomb structure is used, including a porous partition wall that defines a plurality of cells serving as a through channel of fluid (exhaust gas, purified gas) and a circumferential wall located at the outermost circumference. Then, when such a honeycomb structure is used as a filter such as a DPF, a plugging portion is disposed at open ends of predetermined cells at an inflow-side end face of fluid (exhaust gas) and at open ends of the residual cells at an outflow-side end face of the fluid (exhaust gas). With this configuration, the partition wall of the honeycomb structure functions as a filtering layer to trap PM.
When PM in exhaust gas is trapped using such a honeycomb structure, there is a problem that the PM penetrates into the pores of the partition wall and such PM blocks the pores, and then pressure loss of the honeycomb structure increases rapidly.
To suppress such increase of pressure loss, there is proposed a filter that includes a trapping layer to trap PM at the surface of the partition wall so as to prevent penetration of PM into the partition wall, and suppress increase of the pressure loss (e.g., see Non-Patent Document 1).
Conventionally emission control for automobiles has been performed on the basis of the mass of PM, and another emission control on the number of pieces of PM has been studied recently. In that case, PM of small particle diameters has to be trapped reliably. It is known that such PM of small particle diameters is trapped at the surface of the pores present in the filter due to mainly diffusion (e.g., see Non-Patent Document 2).
According to a conventional method for manufacturing a honeycomb structure including a trapping layer, slurry to form the trapping layer (trapping layer forming raw material) is applied to the porous partition wall (partition wall base material), followed by firing to form the trapping layer. Examples of the method for applying slurry to form the trapping layer includes immersion of a honeycomb structure in the slurry to form the trapping layer (trapping-layer forming raw material) or pouring the slurry to form the trapping layer into cells of the honeycomb structure. Then, when a porous membrane having a pore diameter smaller than that of the porous partition wall and having thickness thinner than that of the partition wall is formed at the surface of the porous partition wall, ceramic particles making up the porous membrane have to have a particle diameter that is smaller than the pore diameter of the partition wall. This method, however, has a problem that the slurry to form the trapping layer penetrates into pores of the partition wall (partition wall base material) of the honeycomb structure, and so initial pressure loss when letting exhaust gas pass through the obtained honeycomb structure increases.
When a honeycomb structure is made of cordierite or aluminum titanate, for example, there is a problem that slurry may penetrate into micro-cracks formed at the honeycomb structure or the like, and so the obtained honeycomb structure has a high thermal expansion rate.
Meanwhile there is proposed a method of filling pores of a porous supporting body with “substance capable of being removed later” to block these pores, followed by application of slurry including ceramic particles with a small particle diameter to the surface of the porous supporting body (e.g., see Patent Documents 1 to 3). Such “substance capable of being removed later” includes combustible substance (Patent Document 1), for example. When combustible substance is used, such combustible substance can be combusted for removal through a firing step performed later. Examples of the “substance capable of being removed later” include water and alcohol (Patent Documents 2 and 3). When water or alcohol is used, drying is performed after the application of slurry, whereby such water or alcohol can be removed.
There is proposed still another method of making a ceramic porous membrane (trapping layer) at the surface of a porous supporting body using particulates mainly containing oxides of alumina, zirconia or the like (e.g., see Patent Document 4). Specifically, a ceramic porous membrane is formed at the surface of a porous supporting body made of porous ceramic using paint to form porous membrane, the paint including particulates mainly containing oxides whose average primary particle diameter, tap bulk density and average secondary particle diameter in the paint (average secondary particle diameter in the state of being dispersed in dispersing medium) are controlled, and the dispersing medium mainly containing water, where the viscosity of the paint is controlled to be 2 mPa·s or more and 1,000 mPa·s or less.
There is proposed a further method of spraying slurry to form a trapping layer to a honeycomb formed body, the slurry being prepared by further adding pore former and water to the same material of the honeycomb formed body, whereby the slurry to form the trapping layer is deposited at the partition wall of the honeycomb formed body, followed by drying and firing, whereby a trapping layer is provided at the honeycomb formed body (e.g., see Patent Document 5).
There is proposed a still further method of depositing slurry including an inorganic fibrous material longer than the pore diameter of the porous partition wall and including an adhesive material mainly containing silica or alumina to the surface of the partition wall, followed by drying and firing, whereby a porous membrane (trapping layer) is formed at the surface of the partition wall (e.g., see Patent Document 5).    [Patent Document 1] JP-A-H01-274815    [Patent Document 2] JP-B-S63-66566    [Patent Document 3] JP-A-2000-288324    [Patent Document 4] JP-A-2010-95399    [Patent Document 5] WO 2008/136232    [Non Patent Document 1] SAE Technical Paper 2008-01-0618, Society of Automotive Engineers (2008)    [Non Patent Document 2] SAE Technical Paper 2007-01-0921, Society of Automotive Engineers (2007)